feat(api): API specification v2.21.0

* Scores & Library:
  * `GET /users/{user}/scores`: As planned in 2020, this endpoint has been updated to only return public scores for user community profiles. The endpoint has also a new pagination system and sorting options.
  * `ScoreDetails`: Added `editHtmlUrl`, `instrumentsNames` and `scheduledDeletionDate` properties.
  * `Collection`: Added `contents.scoresCount` property.

* Flat Community profiles:
  * `UserPublic` now includes `likesCount` and `playsCount` properties.

* Flat for Education:
  * `GET /eduResources`:
    * Added options `withoutSubfoldersResources`, `assignmentTypes`, `subjects`, and `grades` to filter content.
    * Added new response headers `X-Total-Assignments-Count` and `X-Total-Folders-Count`.
  * `EduResource`: Added `sharingDescription`, `subjects`, `grades` and `capabilities.canChangePrivacy` properties.
  * `EduResourceFolder`: Added `assignmentsTypes` and `resourcesCount` properties.
  * `Assignment`: Added `restrictPlayNote` and `restrictToAudioTracks` properties.
  * `AssignmentSubmission`: Updated LTI to support LTI 1.1 and 1.3 (`lti.gradeService` property).
  * `ScoreTrack`: Added `purpose` property.

Author: gierschv